1. Field of the Invention
The present invention relates generally to an optical device with optical waveguides having a variable optical attenuation function, and more particularly to an optical device with optical waveguides capable of electrically controlling the attenuation of light passing through an optical waveguide circuit.
2. Description of the Related Art
U.S. Pat. No. 5,956,437 (Japanese Patent Laid-open No. Hei 11-249089) discloses an electrically controllable optical attenuator including a first Mach-Zehnder type optical interference unit and a second Mach-Zehnder type optical interference unit cascaded with each other. The first Mach-Zehnder type optical interference unit includes a pair of optical waveguide arms having different lengths. That is, one of the optical waveguide arms in the first Mach-Zehnder type optical interference unit is longer than the other. Similarly, the second Mach-Zehnder type optical interference unit includes a pair of optical waveguide arms having different lengths. That is, one of the optical waveguide arms in the second Mach-Zehnder type optical interference unit is longer than the other. The longer optical waveguide arm in the first Mach-Zehnder type optical interference unit is provided with phase control means, and the shorter optical waveguide arm in the second Mach-Zehnder type optical interference unit is provided with phase control means. In each optical interference unit, the builtin phase or initial phase of light passing through the longer optical waveguide arm is delayed by xcfx80 or 2xcfx80 from the phase of light passing through the shorter optical waveguide arm.
In the case that the initial phase delay is set to xcfx80, a maximum attenuation, or maximum loss is obtained when the phase delay by the phase control means is 0, whereas a minimum attenuation, or minimum loss is obtained when the phase delay by the phase control means is xcfx80. In the case that the initial phase delay is set to 2xcfx80, a minimum attenuation, or minimum loss is obtained when the phase delay by the phase control means is 0, whereas a maximum attenuation, or maximum loss is obtained when the phase delay by the phase control means is xcfx80.
Silica is used for each optical waveguide arm, and an electric heater is used for each phase control means. When power to be injected into the electric heater is increased, the temperature of the optical waveguide arm on which the electric heater is mounted rises to cause an increase in refractive index. As a result, the phase delay of light passing through the optical waveguide arm on which the electric heater is mounted increases, so that when the initial phase delay is set to xcfx80, the attenuation is decreased, whereas when the initial phase delay is set to 2xcfx80, the attenuation is increased. In the conventional electrically controllable optical attenuator described in the above publication, the attenuation characteristics of the first and second Mach-Zehnder type optical interference units are added together to realize an optical attenuator superior in wavelength flatness.
In the case of using a heater as each phase control means, the phase changes by xcfx80 when a maximum power of about 500 mW is injected into each heater (a total maximum power of about 1 W for the two heaters), and the attenuation can be controlled from a minimum value to a maximum value. In the conventional optical attenuator described in U.S. Pat. No. 5,956,437 mentioned above, substantially the same quantity of energy is injected into each heater to control the attenuation, so that a change in injected power according to the attenuation becomes large. As a result, a change in heat value in the whole of the optical device becomes large. Such a large change in heat value causes a problem that the temperature of the optical attenuator and its peripheral device easily change. Thus, the optical attenuator described in the above publication has the problem that the temperature easily changes with a change in attenuation.
Further, in wavelength division multiplex (WDM) communication, a variable optical attenuator is used as an equalizer for equalizing the powers of a plurality of light sources. In this case, the variable optical attenuator is arranged downstream of each light source, and the powers of the light sources are equalized by setting the attenuation to the light source having the lowest optical power to 0 and attenuating the powers of the other light sources. In the case that the variable optical attenuator is used as such an equalizer for WDM communication, it is operated in a wavelength region where the loss is relatively low. In the conventional electrically controllable optical attenuator described in the above publication, the loss is minimum when the power input into each heater is maximum. Accordingly, in the case of using this conventional optical attenuator as an equalizer for WDM communication, it is operated in a wavelength region where the power consumption becomes substantially maximum. For example, in the case of using this conventional optical attenuator as an equalizer for WDM communication having 32 channels, the maximum power consumption becomes about 32 W.
It is therefore an object of the present invention to provide an optical device with optical waveguides which can electrically control an attenuation with a reduction in power consumption.
It is another object of the present invention to provide an optical device with optical waveguides which can electrically control an attenuation with a reduction in change in heat value according to the attenuation.
It is a further object of the present invention to provide a manufacturing method for an optical device with optical waveguides.
In accordance with an aspect of the present invention, there is provided an optical device comprising a first Mach-Zehnder type optical interference unit including a first input optical waveguide, a first input 3-dB optical coupler optically connected in tandem with said first input optical waveguide, first and second interference optical waveguide arms optically connected in tandem with said first input 3-dB optical coupler opposite to said first input optical waveguide, said second optical waveguide arm having a length shorter than that of said first optical waveguide arm, a first output 3-dB optical coupler optically connected in tandem with said first and second interference optical waveguide arms, and a first output optical waveguide optically connected in tandem with said first output 3-dB optical coupler opposite to said first and second interference optical waveguide arms; a second Mach-Zehnder type optical interference unit including a second input optical waveguide, a second input 3-dB optical coupler optically connected in tandem with said second input optical waveguide, third and fourth interference optical waveguide arms optically connected in tandem with said second input 3-dB optical coupler opposite to said second input optical waveguide, a second output 3-dB optical coupler optically connected in tandem with said third and fourth interference optical waveguide arms, and a second output optical waveguide optically connected in tandem with said second output 3-dB optical coupler opposite to said third and fourth interference optical waveguide arms; first phase control means provided on said first interference optical waveguide arm; and second phase control means provided on any one of said third and fourth interference optical waveguide arms; the lengths of said third and fourth interference optical waveguide arms being adjusted so that the phase difference of light having a given wavelength passing through said third and fourth interference optical waveguide arms becomes 2nxcfx80 where n is an integer greater than or equal to 0; said first and second Mach-Zehnder type optical interference units being optically connected in tandem with each other.
The lengths of said first and second interference optical waveguide arms are adjusted so that the phase difference of light having a given wavelength passing through said first and second interference optical waveguide arms becomes (2n+1+xcex1)xcfx80 where n is an integer greater than or equal to 0 and xcex1 is a number greater than or equal to 0 and less than 1. Preferably, said first phase control means comprises a first heating element, and said second phase control means comprises a second heating element.
Preferably, the optical device further comprises a first drive circuit for driving the first heating element, a second drive circuit for driving the second heating element, and a controller for controlling the first and second drive circuits. The controller drives the second drive circuit to control it so that predetermined initial electric energy is preliminarily applied to the second heating element. The controller further controls the first and second drive circuits so that when increasing electric energy to be applied to the first heating element, electric energy to be applied to the second heating element is simultaneously decreased, whereas when decreasing the electric energy to be applied to the first heating element, the electric energy to be applied to the second heating element is simultaneously increased.
Preferably, the controller controls the first and second drive circuits so that an increase in the electric energy to be applied to the first heating element becomes equal to a decrease in the electric energy to be applied to the second heating element when decreasing the electric energy from the second heating element simultaneously with increasing the electric energy to the first heating element. Alternatively, the controller controls the first and second drive circuits so that an increase in the electric energy to be applied to the first heating element becomes larger than a decrease in the electric energy to be applied to the second heating element when decreasing the electric energy from the second heating element simultaneously with increasing the electric energy to the first heating element.
According to the present invention, the wavelength flatness can be improved, and the sum of powers to be supplied to the two heating elements can be reduced as compared with the prior art. Furthermore, a change in the sum of powers to be supplied to the two heating elements can also be reduced as compared with the prior art, thereby reducing a change in heat value in the whole of the optical device having a variable optical attenuation function. In the case of using the optical device according to the present invention as an equalizer for WDM communication, the optical device is generally operated in a wavelength region where the power consumption becomes minimum, thereby effecting a reduction in power consumption.
In accordance with another aspect of the present invention, there is provided an optical device comprising a first Mach-Zehnder type optical interference unit including a first input optical waveguide, a first input 3-dB optical coupler optically connected in tandem with said first input optical waveguide, first and second interference optical waveguide arms optically connected in tandem with said first input 3-dB optical coupler opposite to said first input optical waveguide, said second optical waveguide arm having a length shorter than that of said first optical waveguide arm, a first output 3-dB optical coupler optically connected in tandem with said first and second interference optical waveguide arms, and a first output optical waveguide optically connected in tandem with said first output 3-dB optical coupler opposite to said first and second interference optical waveguide arms; a second Mach-Zehnder type optical interference unit including a second input optical waveguide, a second input 3-dB optical coupler optically connected in tandem with said second input optical waveguide, third and fourth interference optical waveguide arms optically connected in tandem with said second input 3-dB optical coupler opposite to said second input optical waveguide, a second output 3-dB optical coupler optically connected in tandem with said third and fourth interference optical waveguide arms, and a second output optical waveguide optically connected in tandem with said second output 3-dB optical coupler opposite to said third and fourth interference optical waveguide arms; first phase control means provided on said second interference optical waveguide arm; and second phase control means provided on any one of said third and fourth interference optical waveguide arms; the lengths of said first and second interference optical waveguide arms being adjusted so that the phase difference of light having a given wavelength passing through said first and second interference optical waveguide arms becomes 2mxcfx80 where m is an integer greater than 0; the lengths of said third and fourth interference optical waveguide arms being adjusted so that the phase difference of light having a given wavelength passing through said third and fourth interference optical waveguide arms becomes 2nxcfx80 where n is an integer greater than or equal to 0; said first and second Mach-Zehnder type optical interference units being optically connected in tandem with each other.
In accordance with a further aspect of the present invention, there is provided a manufacturing method for an optical device with optical waveguides, comprising the steps of uniformly forming an under-cladding layer on a substrate; uniformly forming a core layer on said under-cladding layer; etching said core layer to form a core and a core base wider than said core and continuous to said core; uniformly forming an over-cladding layer on said under-cladding layer so as to cover said core and said core base; removing said over-cladding layer on said core base, a part of said core, and a part of said core base and said under-cladding layer by etching to form an optical component mounting surface and expose an end surface of said core; and mounting an optical component on said optical component mounting surface so that said optical component is optically coupled to said end surface of said core.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.